The binding of substrates to adenylate kinase (AK) and the SAM II riboswitch (riboswitch) is accompanied by significant compaction of the respective protein and RNA molecules. Equilibria between less compact and more compact conformations of macromolecules may also be influenced by excluded volume effects in high concentrations of “inert” cosolutes. The conformation of AK in the presence of the substrate ATP and the “inert” osmolyte trimethylamine-N-oxide (TMAO) has been characterized over a wide range of composition by time-resolved fluorescence resonance energy transfer. The conformation of riboswitch in the presence of the substrate S-adenosyl methionine (SAM) and TMAO has been characterized over a wide range of composition by circular dichroism spectroscopy and measurement of diffusion. In both systems, it has been established that high concentrations of TMAO have a substantial compaction effect upon the macromolecule deriving from excluded volume, that facilitates substrate binding to a significant extent. TMAO additionally appears to have a mild anti-compaction effect upon riboswitch at low concentration, due to preferential interaction with the less-compact conformation(s) of the RNA.